Of the four refractive interfaces of the eye, the anterior surface of the cornea provides most of the refractive power of the eye. Therefore, various surgical techniques have been developed which change the curvature of the cornea in order to treat ophthalmic conditions involving errors of refraction such as myopia and hyperopia. These techniques include keratotomy, keratomileusis by a freezing process, automated lamellar keratomileusis (ALK), photo-reactive keratomileusis (PRK), laser-assisted in situ keratomileusis (LASIK), laser intrastromal keratomileusis, laser epithelial keratomileusis (LASEK), conductive keratoplasty (CK), and scleral resection (see published US Patent Application 2003/0139737; U.S. Pat. Nos. 5,144,630; 5,520,679; 5,484,432; 5,489,299; 5,722,952; 5,465,737; 5,354,331; 5,529,076, 6,258,082; 6,263,879; each of which is incorporated herein by reference). All of these techniques work by using various techniques to change the curvature of the cornea, but they are limited by how much refractive error can be corrected and the type of patients who can be treated using these techniques (e.g., in some patients the cornea is too thin to safely utilize techniques which would further thin the cornea). Some of the techniques involve making incisions in the cornea with a diamond knife and/or ablating areas of the cornea thereby increasing the risk of infection or other complications. These techniques also largely depend on the dexterity of the surgeon performing the proocedure, his or her surgical experience, and his experience performing laser ablations (e.g., with a Er:YAG (at 2.94 microns), Ho:YAG laser (at about 2 microns); Raman-shifted solid state laser (at 2.7-3.2 microns), or optical parametric oscillation (OPO) laser (at 2.7-3.2 microns).
Even more modern techniques are limited by their ability to cut corneal or sclera tissue with the desired precision causing a small, or even moderate, amount of refractive error to remain after the procedure and not allowing one to achieve the desired vision for near and for far in one single surgical procedure. The remaining refractive error may also be irregular making it more difficult to correct in the future. When one can not meet the visual demands that the patient requires, the ophthalmologist must resort to additional methods to correct the remaining refractive error. This is usually done by prescribing eye glasses, prescribing contact lenses, or performing a second surgical procedure (commonly known as a “retouch”). Therefore, limitations on the correction of refractive error using these techniques are significant, and the risk of having uncorrectable vision even with a secondary measure is considerable.
In addition, attempts to treat presbyopia using these techniques have also had very limited success. Presbyopia, also known as short arms disease, is a lack of lens accommodation, which prevents the eye from changing its focus. This phenomenon eventually occurs in all individuals over the age of forty. Accommodation allows an individual to see nearby objects by causing both eyes to converge on a near focal point, the pupil to shrink (myosis), and the lens to increase its dioptric power, thereby increasing its curvature in order to focus the image of nearby objects on the retina. Typically, young children have a total accommodation of 14 diopters. As a person ages, the lens of the eye becomes larger, thicker, and less elastic. These changes in the lens are largely due to the progressive denaturation of proteins in the lens. As the ability of the lens to change shape decreases, the power of accommodation decreases from approximately 14 diopters in young children to less than 2 diopters at the age 45 to 50 and to about zero at age 70. Once a person reaches the state of presbyopia, the eye remains focused permanently at an almost constant distance, which is largely determined by the physical characteristics of the individual's eye. The eye can no longer accommodate to see both near and far requiring an older person to wear bifocal glasses with the upper segment for seeing far and the lower segment for seeing near.
This general view of accommodation and presbyopia also does not tale into account other aspects of the visual system. For example, this view does not take into account the higher cognitive functions necessary to orchestrate the eyes, the muscular system, and the brain including the visual cortex in the process of accommodation. The monovision techniques described above (e.g., the myopization of one eye, LASIK monovision), the different techniques that cause positive areas in the central zone of the cornea by malting changes in the peripheral curvature, and the sclera resection or implants to change the scleral rigidity, cilliary muscle, and zonule, and increase the accommodation power of the lens among other more invasive techniques have had very limited success intreating presbyopia. These disappointing results may stem from a variety of sources including the lack of full understanding of the physiological behavior of the eye and its connections with the brain, the nervous system, and the muscular system, the imprecise measurement of the refractive power of the cornea and lens, and the lack of precision in surgical techniques performed by human surgeons.
Ophthalmologists have begun to use sophisticated equipment to measure various parameters of the eye in order to treat presbyopia. However, even the most sophisticated measurements are just approximations due to the fact that the cornea and other parts of the eye are similar to a fingerprint in that there are numerous variations which cannot be adequately described by a finite set of parameters. Also, it is impossible to precisely know how the cornea, lens, retina, and other parts of the visual system will react after surgery under different conditions (e.g., near and far visual stimuli). Furthermore, it is impossible to know how the cornea will heal after refractive surgery (e.g., the final radius of curvature).
The limitations on the existing treatments of presbyopia stem from the fact that these techniques consider only one anatomical region of the eye (i.e., the cornea or the lens). Any correction of near vision in turn causes the far vision of the subject to diminish. In addition, these current techniques model the eye using, among others, Gullstrand's model of the eye which neglects the individuality and uniqueness of each subject's eyes. For example, the ocular globe is not a perfect sphere. Although there are many mathematical models of the eye and its components used in calculating corneal power and the power of the globe (e.g., ray tracing), Gullstrand's model is probably the most popular.
Therefore, a need remains for a successful, non-invasive treatment of presbyopia. Presumably, this treatment could also be used to treat other ophthalmic conditions involving refractive errors including myopia, hyperopia, and astigmatism.